A waveband used by an optical communications system is generally classified as a 1.3 .mu.m band and a 1.5 .mu.m band. For an optical amplifier for the 1.3 .mu.m band, there has been known a structure in which an optical amplifying fiber doped with Pr (praseodymium) is pumped by a laser beam of a 1.02 .mu.m band. In contrast, for an optical amplifier for the 1.5 .mu.m band, there has been known a structure in which an optical fiber doped with Er (erbium) is pumped by a laser beam of a band ranging from 1.47 to 1.58 .mu.m or a 0.98 .mu.m band.
FIG. 3 shows a block diagram schematically showing the conventional configuration. Reference numeral 50 designates a 2.times.1 (two inputs/one output) wavelength Division Multiplexing optical coupler. Signal light to be optically amplified enters into one of two input terminals of the optical coupler, and output light from a pumping laser 52 enters into the other input terminal. The optical coupler 50 multiplexes both input signals in wavelength domain, and the thus-multiplexed light enters into an optical amplifying fiber 56 via a matching connecting member 54. The optical amplifying fiber 56 optically amplifies the signal light by means of the pumping light output from the pumping laser 52. The signal light optically amplified by the optical amplifying fiber 56 is supplied to a signal transmission optical fiber 60 via a matching connecting member 58. The matching connecting members 54, 58 are provided for reducing the reflection of light when the core diameter and refractive index of the optical amplifying fiber 56 differ from those of the optical fiber connected to the output terminal of the optical coupler 50 and those of the signal transmission optical fiber 60. Needless to say, if there is no need for such optical matching, the matching connecting members 54, 58 are omitted.
In FIG. 3, the optical amplifying fiber 56 is pumped from the forward end. In contrast, the structure wherein the optical amplifying fiber is pumped from the backward end (more specifically, the pumping laser 52 and the WDM optical coupler 50 are located in a stage subsequent to the optical amplifying fiber 56 in such a way that pumping light travels through the optical amplifying fiber in the direction opposite that in which the signal light travels), and the structure wherein the optical amplifying fiber is pumped from both ends are well known. The latter structure has the advantages that the output power from the respective pumping lasers may be rather small, and that the amplification gain can be made uniform in the longitudinal direction of the optical amplifier.
In the optical amplifier used for the 1.3 .mu.m band, the optical amplifying fiber 56 is an optical fiber doped with Pr (praseodymium) ions, and the pumping laser 52 is a laser which is laser-oscillated in the 1.02 .mu.m band. In the optical amplifier used for the 1.5 .mu.m band, the optical amplifying fiber 56 is an optical fiber doped with Er ions, and the pumping laser 52 is a laser which is laser-oscillated in the 1.58 .mu.m band or the 0.98 .mu.m band. Thus, the optical amplifier for the 1.3 .mu.m band and the optical amplifier for the 1.5 .mu.m band are essentially the same in structure and differ from each other only in material used for doping an optical amplifying medium and a pumping wavelength.
The conventional optical amplifier for the 1.3 .mu.m band provides a small gain. For example, even in the case of both-end pumping, in order to obtain gain of 20 dB, 1.02 .mu.m-band light having a power as high as about 700 mW must be input to a Pr-doped optical fiber from both ends thereof. Accordingly, such an optical amplifier cannot be applied to an actual optical communications system. Further, a 1.02 .mu.m-band laser is considerably special and hence expensive.
In contrast, the conventional optical amplifier for the 1.5 .mu.m band does not present problems such as those mentioned previously. However, pumping light is merely input into an Er-doped optical fiber, and hence light which has not been absorbed by the Er-doped optical fiber is discarded and never used, resulting in inefficient pumping. Further, the intensity of pumping light is greatest at the entrance of an optical amplifying fiber and becomes gradually attenuated as the pumping light travels through the optical amplifying fiber, so that the optical amplification gain decreases and noise increases accordingly. More specifically, there is a problem that the gain varies in the longitudinal direction of the optical amplifying fiber.